Oil platforms are often used in offshore oil and gas production. More recently, processing facilities are being relocated to the ocean floor. Such subsea installations may be located in considerable water depths, for example in a depth of more than 1.000, 2.000 or even more than 3.000 meters. At such water depths, corresponding pressures of about 100, 200 or 300 bar, respectively, prevail. The devices forming part of such subsea installation do accordingly need to be capable of handling such high ambient pressures.
One possibility of handling high pressures is the use of a pressure resistant enclosure which maintains a close to atmospheric pressure inside, thus enabling the use of standard topside components. A further possibility is the use of a pressure compensated enclosure. Such enclosure generally comprises a pressure compensation system, or pressure compensator, which balances the pressure inside the enclosure to the pressure prevailing in the ambient seawater. Due to the large volume changes experienced by gases when increasing the pressure, the pressure compensated enclosure is generally filled with a dielectric liquid, thus keeping the volume changes which the liquid experiences and which the pressure compensator needs to compensate relatively low.
When using such pressure compensated enclosure, it is desirable to know which status the pressure compensator has at any point in time and whether the pressure compensator operates correctly. In conventional subsea devices, only pressure sensors are available for determining the pressure inside the subsea device, which may indicate the functioning of the pressure compensator. A particular problem is that due to the pressure and temperature differences between the topside and the ocean floor, relatively large volume differences can occur, resulting in large variations of the pressure compensator. Large variations are difficult to measure with high precision, and furthermore, the sensors available are generally not operable in the environment prevailing inside a pressure compensated subsea device, i.e. within a dielectric liquid at high pressures. It is thus desirable to measure relatively large displacements with high accuracy in such difficult environment.